Texas came first, and it came apart from Mexico before the US ever formally took it. American settlers in the Mexican province of Texas fought a revolution against the centralist government of President Antonio López de Santa Anna from October 1835 to April 1836, winning independence and creating the Republic of Texas -- a separate, internationally recognized nation that existed on its own for nearly a decade.^[1] The US didn't annex Texas until Congress did it by joint resolution, and Texas became the 28th state on December 29, 1845 -- nine years after it had already stopped being Mexican and become something else first.^[2]

The rest of the transfer happened in one war, on a manufactured pretext, and one treaty. President James K. Polk moved US troops into disputed border territory while simultaneously pursuing negotiations to acquire California and New Mexico from Mexico outright. When a skirmish between Mexican and American troops broke out in the disputed zone in April 1846 -- the Thornton Affair -- Polk used it to secure a congressional declaration of war.^[3] The Mexican-American War ran from 1846 to 1848 and ended with the Treaty of Guadalupe Hidalgo, signed February 2, 1848: the United States paid $15 million and assumed more than $3 million in claims US citizens held against Mexico, in exchange for more than 525,000 square miles of Mexican territory -- California, Nevada, Utah, most of Arizona and New Mexico, and parts of Colorado and Wyoming, with the new border drawn along the Rio Grande and the Gila River.^[4] Mexico lost nearly half its national territory in that single document. A second, smaller purchase followed in 1853-54 -- the Gadsden Purchase, another $10 million for a strip of what's now southern Arizona and New Mexico, driven by the US wanting a viable southern route for a transcontinental railroad.^[5]

The treaty's text promised the people already living there full protection -- and the Senate cut that promise out before the ink on ratification was even dry. Under Guadalupe Hidalgo, Mexican citizens remaining in the ceded territory automatically became US citizens, and Article X of the treaty as negotiated guaranteed protection of existing Spanish and Mexican land grants -- including extra time for Texas grantees whose conditions had been disrupted by the fighting. When the Senate ratified the treaty on March 10, 1848, by a vote of 34 to 14, it did so only after striking Article X outright, at President Polk's own urging.^[6] Not lost to interpretation later -- removed from the actual document at the moment of approval.

What replaced that protection put the burden of proof on exactly the people the treaty claimed to protect. The California Land Act of 1851 required anyone holding a Spanish- or Mexican-era land grant to prove their title before a federal Board of Land Commissioners -- inverting the treaty's own promise of automatic protection into an adversarial legal process the landholders had to win, in a language and a court system most of them had never navigated.^[7] The commission ultimately confirmed 604 of 813 claims filed, but nearly every single one still went through the courts regardless, taking an average of seventeen years to resolve -- and the cost of lawyers, translators, and surveyors forced many Californio landholders to sell off portions of their property, or trade it outright, just to pay their own legal bills.^[7] Combined with a new state tax system, drought, and constant conflict with squatters, that decades-long legal grind dispossessed most of California's original Mexican landholding families of their land within a single generation -- not through any single seizure, but through the accumulated cost of defending a right the treaty had already granted them. In 1889, the US Supreme Court settled the underlying question directly: in Botiller v. Dominguez, an 8-0 ruling held that an act of Congress overrides a treaty, and that federal courts had no power to enforce Guadalupe Hidalgo's original protections against a later, contradicting US statute.^[8]

Why does this matter? This is the same gap who created whom traces through the states and the federal government and held to its own word traces through the founders themselves -- a formal, written promise, and a separate, later, quieter mechanism that determines how much of it actually survives contact with the people who have to enforce it. Texas and California aren't places America expanded into empty land. They're places where an entire existing legal and cultural order -- Spanish and Mexican land grants, citizenship, property rights, all guaranteed in writing -- got absorbed, and then spent the following decades finding out, one court case at a time, how much of that guarantee Congress actually intended to honor.